Open YouTube Studio
Go to studio.youtube.com and sign in with the channel you want to edit.

How-to
Tags live in YouTube Studio's advanced settings, not the upload screen. Here are the five steps — it takes under a minute, on any published or new video.
Go to studio.youtube.com and sign in with the channel you want to edit.
In the left menu, click Content, then select the video you want to add tags to.
On the video details page, scroll down and click Show more to reveal advanced settings.
In the Tags field, type each tag and press Enter or comma. Lead with your exact target keyword; stay under the 500-character limit.
Click Save in the top-right. Tags apply immediately — no re-upload needed.
In YouTube Studio → Content → your video → Show more → the Tags field. Tags are not on the main upload screen; you have to expand the advanced settings.
Yes. You can edit tags on any published video at any time — just save and the change applies immediately.
500 characters total across all tags. Focus on a tight, relevant set rather than filling the whole limit.